1. Field of the Invention
The present invention relates to a motor control device that converts AC power supplied from a three-phase AC power supply into DC power, outputs the DC power to a DC link, further converts the DC power into AC power for the driving of a motor, and supplies the AC power to the motor, and, specifically, to a motor control device having a protective unit of a charging resistor used for the charging of a smoothing capacitor provided to a DC link.
2. Description of the Related Art
In a motor control device that drives a motor in a machine tool, a press-forging machine, an injection molding machine, an industrial machine, or various robots, AC power input from an AC power supply side is converted into DC power once, the DC power is further converted into AC power, and this AC power is used as driving power of motors provided for each driving axis.
FIG. 7 is a diagram illustrating the configuration of a general motor control device. A motor control device 100 includes a rectifier 11 that rectifies AC power from a three-phase AC input power supply 3 and outputs DC power, and inverters 12 that are connected to a DC link, which is a DC side of the rectifier 11, and convert the DC power output from the rectifier 11 into AC power with a desired voltage and a desired frequency to be supplied as driving power of motors 2 or convert AC power regenerated from the motors 2 into DC power, and controls the speed and torque of the motors 2 connected to AC sides of the inverters 12 or the position of a rotator. In addition, in FIG. 7, an AC reactor of a three-phase AC input side of the rectifier 11 is indicated by L.
The inverters 12 having the same number as that of the motors 2 respectively provided corresponding to a plurality of driving axes are connected in parallel to individually supply driving power to each motor 2 and to drive and control each motor 2. Smoothing capacitors 32 are respectively provided to DC input sides of the inverters 12. In addition, in FIG. 7, since the number of motors 2 is 1 for simplification, the number of inverters 12 is one. On the other hand, one rectifier 11 is provided to a plurality of inverters 12 for the purpose of reducing the cost and space occupied by the motor control device 100 in many cases.
The smoothing capacitor 32 needs to be charged until the driving of the motor 2 is started (i.e., a power conversion operation is started by the inverter 12) immediately after the motor control device 100 starts to operate. Hereinafter, the charging of the smoothing capacitor 32 before the driving of the motor 2 is started will be referred to “initial charging”. In the state in which no energy has been accumulated in the smoothing capacitor 32 at the time of the start of the initial charging, a large inrush current flows through the rectifier 11. Specifically, as the capacity of the smoothing capacitor 32 is large, a larger inrush current is generated. As a countermeasure of the inrush current, it is general to provide an initial charging unit 13 between the rectifier 11 and the smoothing capacitor 32, or to the three-phase AC input side of the rectifier 11. The example illustrated in FIG. 7 illustrates the case in which the initial charging unit 13 is provided between the rectifier 11 and the smoothing capacitor 32. Although not illustrated in the drawing, for example, in the case in which a plurality of inverters 12 are connected in parallel, the inverters 12 have a parallel connection relation for the smoothing capacitor 32 in response to the case. However, the case in which the initial charging unit 13 is provided between the rectifier 11 and the smoothing capacitor 32, one initial charging unit 13 is provided between the rectifier 11 and these smoothing capacitors 32. Furthermore, for example, although not illustrated in the drawing, when the initial charging unit 13 is provided to the three-phase AC input side of the rectifier 11, the initial charging unit 13 is provided to at least two phases of the three phases of the three-phase AC input side of the rectifier 11, regardless of the number of connected inverters 12.
The initial charging unit 13 has a switch unit 33 and a charging resistor 31 connected in parallel to the switch unit 33. The switch unit 33 is opened (turned off) only during an initial charging period of the smoothing capacitor 32 immediately after the motor control device 100 starts to operate, and maintains a closed circuit state (a turn-on state) during a typical operation period in which the motor control device 100 drives the motor 2. In more detail, the switch unit 33 is opened (turned off) during the initial charging period until the driving of the motor 2 is started immediately after the motor control device 100 starts to operate, so that the DC power output from the rectifier 11 flows into the smoothing capacitor 32 through the charging resistor 31 and thus the smoothing capacitor 32 is charged. When the smoothing capacitor 32 is charged up to a predetermined voltage, the switch unit 33 is closed (turned on), so that the initial charging operation is completed. Thereafter, the inverter 12 starts a power conversion operation to supply the motor 2 with driving power, so that the motor 2 is driven on the basis of the driving power.
As described above, during the initial charging period of the smoothing capacitor 32, since the switch unit 33 is opened (turned off), the DC power output from the rectifier 11 flows through the charging resistor 31 and is consumed in the charging resistor 31 as heat to a certain degree, so that the generation of an excessive inrush current is suppressed during the initial charging period. However, when heat is excessively generated by a current flowing through the charging resistor 31, the charging resistor 31 is fused. In general, the charging resistor 31 has instantaneous load tolerance (hereinafter, simply referred to as “tolerance”) defined as heat capacity tolerable against fusing. When a DC current generating heat capacity equal to or more than the instantaneous load tolerance continuously flows through the charging resistor 31, the charging resistor 31 is fused.
Since the DC current flowing through the charging resistor 31 during the initial charging period depends on the capacity of the smoothing capacitor 32, and heat capacity generated in the charging resistor 31 depends on the DC current flowing through the charging resistor 31, the maximum capacity of the smoothing capacitor 32 installable in the motor control device 100 is decided in response to the tolerance of the charging resistor 31. At the time of design of the motor control device 100, a designer typically selects the smoothing capacitor 32 in a range in which the maximum capacity is not exceeded in consideration of the relationship between the tolerance of the charging resistor 31 and the maximum capacity of the smoothing capacitor installable under the tolerance.
When a designer erroneously selects a smoothing capacitor 32 having a capacity not satisfying the aforementioned design condition (i.e., a smoothing capacitor 32 having a capacity exceeding the maximum capacity of the smoothing capacitor 32 decided in response to the tolerance of the charging resistor 31), a DC current equal to or more than the tolerance flows through the charging resistor 31, resulting in the occurrence of the abnormality of an initial charging unit such as fusing of the charging resistor 31. For example, when a designer designs the motor control device 100 provided with a plurality of inverters 12 for the purpose of driving a plurality of motors 2 or when the smoothing capacitor 32 is exchanged into a smoothing capacitor having a large capacity or a new smoothing capacitor 32 is added in maintenance later and the like, the total capacity of the capacities of a plurality of smoothing capacitors 32 exceeds the maximum capacity of the smoothing capacitor 32 decided in response to the tolerance of the charging resistor 31 (hereinafter, such a state will be simply referred to “excess in the capacity of the smoothing capacitor 32”).
Furthermore, even though the motor control device 100 satisfying the aforementioned design condition is provided, for example, when the DC link is short-circuited by failure of a switching element and the like of the rectifier 11 or the inverter 12, a DC current equal to or more than the tolerance flows through the charging resistor 31 during the initial charging period, resulting in the occurrence of the abnormality of the initial charging unit such as fusing of the charging resistor 31.
As described above, as a main factor causing a DC current equal to or more than the tolerance to flow through the charging resistor 31, there are excess in the capacity of the smoothing capacitor 32, and short-circuit of the DC link by failure of the switching element and the like of the rectifier 11 or the inverter 12. Hereinafter, the case in which the initial charging unit 13 is provided between the rectifier 11 and the smoothing capacitor 32 will be described. However, even in the case in which the initial charging unit 13 is provided to the three-phase AC input side of the rectifier 11, a problem such as fusing of the charging resistor 31 occurs in a similar manner.
As disclosed in Japanese Unexamined Patent Publication No. 2000-152643, as a conventional art for detecting the abnormality of an initial charging unit, there is a technology in which when a value obtained by temporally integrating an output current of the initial charging unit is compared with a value obtained by multiplying the capacity of a capacitor connected to both ends of a DC side of an inverter by the voltage of the capacitor and a difference equal to or more than a predetermined setting value exists between these two values, it is determined that there is abnormality in the initial charging unit and a charging operation circuit is disconnected.
Furthermore, as disclosed in Japanese Unexamined Patent Publication No. 2013-205257, there is a technology in which the capacity of a capacitor connected in parallel on the input side of a load and a time constant specified from a resistance value of a precharge circuit are detected from voltage rise characteristics of a precharged capacitor, and the abnormality of the precharge circuit or the capacitor is determined on the basis of the detected time constant.
According to the aforementioned conventional arts, it is possible to detect the abnormality of the initial charging unit, which is caused by the short-circuit of the DC link by failure of the switching element and the like of the rectifier 11 or the inverter 12, but there is a problem that it is not possible to detect the abnormality of the initial charging unit, which is caused by excess capacity of the smoothing capacitor 32. As described above, when a designer designs the motor control device 100 provided with a plurality of inverters 12 for the purpose of driving a plurality of motors 2 or when the smoothing capacitor 32 is changed into a smoothing capacitor having a large capacity or a new smoothing capacitor 32 is added in maintenance later and the like, the total capacity of the capacities of a plurality of smoothing capacitors 32 may exceed the maximum capacity of the smoothing capacitor 32 decided in response to the tolerance of the charging resistor 31. Even though the smoothing capacitors 32 are connected in parallel or the smoothing capacitor 32 is exchanged into a smoothing capacitors having a different capacity, since there is no change in a DC voltage value of both ends of the smoothing capacitor 32, it is not possible to detect the abnormality of the initial charging unit, which is caused by “excess capacity of the smoothing capacitor 32” in the aforementioned conventional arts. When it is not possible to detect the abnormality of the initial charging unit, it is not possible to appropriately protect the initial charging unit (specifically, the charging resistor).